1. Field of the Invention
The invention relates to a method and an apparatus for removing colorants from PET-flake which can be obtained by shredding commercially available used PET bottles.
2. Description of Relevant Art
PET is the common abbreviation for “polyethylene terephthalate” or more precisely “poly(ethylene terephthalate)” (CAS: 25038-59-9). PET is a thermoplastic polyester and widely used for food containers and bottles, e.g. for water, carbonated soft drinks, beer or the like. Pure PET is transparent for visible light with wavelengths between 380 nm and 780 nm and thus colorless. For aesthetic reasons and as well as to protect the liquids to be handled by PET bottles from degrading when subjected to light, the bottles are often colored by organic colorants and/or pigments.
Large amounts of used bottles are to be recycled. Various chemical recycling techniques are known, in particular glycolysis, methanolysis, hydrolysis, and saponification. However, according to the Petcore PET knowledge centre these methods are unable to remove colors from the PET feedstreams (www.petcore.org/content/processing of 2 May 2013)
EP 1 153 070 B1 suggests to contact PET with a glycol, e.g. ethylene glycol (briefly “EG”) to thereby obtain oligomers and monomers of the PET, in particular bis-(2-hydroxyethyl) terephthalate (briefly “BHET”, CAS: 959-26-2) in an agitated reactor vessel at a temperature of about 150° C. to 300° C. and an absolute pressure of 0.5 to 3.0 bars. The ratio of EG to dicarboxylic acid is greater than 1 to 5 total glycol units to total dicarboxylic acid units. Lower density contaminants form a distinct top layer in the reactor vessel and are separated from a lower layer containing a remainder of a glycolysis reaction mixture. Remaining immiscible contaminants are removed by filtration or straining.
PET bottle recycling is addressed by the European Patent EP 1 437 377 B1. First PET bottles are unpacked, steel and aluminum residues are removed and the bottles are shredded. Subsequently, non-PET polymers are separated by winnowing and float-sink separation. The such obtained PET-flake are depolymerized by charging them into EG at 175-190° C. at 0.1-0.5 MPa to thereby obtain BHET which is later subjected to an ester interchange reaction for forming crude dimethyl terephthalate (briefly “DMT”, CAS: 120-61-6) and EG. DMT and EG can be separated, purified and again used as monomers in the polymer industry.
EP 1 914 270 suggest to recover colorants from dyed polyester fiber as used in PET-fabric by a dye extraction using ethylene glycol.